Thermostats have been used for many years as temperature sensitive switches which control heating and/or cooling equipment for conditioning a space in which a thermostat, or a temperature sensor connected to the thermostat, is placed. In the well known manner, a simple thermostat can be adjusted to establish a temperature set point such that, when the temperature in the conditioned space reaches the set point, the thermostat interacts with the heating and/or/cooling equipment to take suitable action to heat or cool the conditioned space as may be appropriate for the season.
Modern thermostats, which take advantage of the ongoing rapid advances in electronic technology and circuit integration, have many features which provide more precise supervision of the heating and/or cooling equipment to achieve more economical and more comfortable management of the temperature of a conditioned space. Many modern thermostat include a real time clock, a memory and a data processor to run a process control program stored in the memory to accurately measure the temperature of a temperature sensor disposed in the conditioned space and to send control signals to the heating and/or cooling equipment to closely control the temperature of the conditioned space. The use of programmed thermostat systems permits anticipating and minimizing hysterisis or overshoot of the temperature in the conditioned space. In addition, the program can specify different set points at different times of the day and days of the week and may also include a “vacation” mode which employs different set points when the conditioned space is not occupied for an extended period.
Most such modern thermostats are programmable by a user. Typically, a programmable thermostat incorporates a tactile user interface with various buttons (or representations of buttons on a touch screen) to be touched in a precise sequence to program set points (which may vary with the time of day and day of the week) for programmable time periods and which may include both normal and vacation modes. The programming sequence may be followed on a display, typically a liquid crystal display, and when the programming steps have been completed, the thermostat is switched the immediately desired operational mode.
The purpose of a vacation mode is to maintain the temperature of a conditioned space which will be unoccupied for a period at a value which will substantially lower the cost of energy to heat/cool the conditioned space but which ensures that the temperature will not fall into a range at which damage to the premises, for example, freezing pipes, will occur. The operation of programmable thermostats in the vacation mode is straightforward and well known. A separate program from normal operation is entered by whatever procedure is provided in a given thermostat. The vacation mode may include one or more periods a day for which set point temperatures may be entered. In cold climates, a heating temperature set point will be selected by a user to be suitable, say 45° F., such that freezing of water pipes will not take place. If, for example, self sufficient pets are left behind for a few days, a higher temperature, say 60° F., might be selected. The choice is within the purview of the user. Similarly, in hot climates, a user might select a temperature of, say, 90° F. or even higher to effect substantial savings in the cost of energy for cooling, but not so high as to cause damage to items in the space. Again, if, for example, pets are left behind, a more comfortable temperature of 80° F. or so might be selected.
Some programmable thermostats incorporating a vacation mode of operation also provide for storing the date and even time of day that the occupant(s) of the conditioned space will return such that operation of the space conditioning equipment may automatically earlier be returned to a normal mode of operation in anticipation of the return of the occupant(s), thus ensuring a comfortable temperature in the conditioned space at that time.
Most commonly, power to a programmable thermostat is supplied from the controlled space conditioning equipment, and either a battery or very high capacity capacitor (“supercap”) provides backup power to “ride out” equipment power failure (“outage”) until power is restored. However, these are not fail safe expedients. Batteries must be changed at intervals to ensure that the thermostat will continue to hold its time and setting throughout an outage, but this maintenance task is often ignored or followed sporadically. Supercaps, on the other hand, are susceptible over time to developing leakage which greatly diminish their charge-storing capacity; i.e., they may develop a resistance across their plates which is sufficiently low (even though the resistance value might ordinarily thought to be very high in other circuits) that a charge stored during normal operation discharges to an insufficient level during a power outage before power is restored.
In either instance, the real time clock loses its current time and date values. This state, particularly if the outage occurs while the thermostat is operating in the vacation mode, can have drastic consequences and, at best, can be a significant inconvenience. If, upon the restoration of power, the thermostat enters a programming mode (as if the thermostat were newly installed and because all programming and/or current time and date information has been lost), the normally conditioned space will not be heated or cooled as the case may be such that the reasons set forth above for using the vacation mode will not be met. Even if the thermostat has the capability of restarting in its operational mode at the time of the outage (e.g., if the processor memory includes non-volatile random access memory for storing changeable information such as set up and mode data and can restart the real time clock at, say, some default date), the thermostat real time clock will not have the correct time or date such that, for example, in the vacation mode, the system will not correctly anticipate the return of occupants and thus will not necessarily earlier resume normal operation to render the space comfortable by the time the occupants return.
It will therefor be apparent to those skilled in the art that it would be highly desirable to provide a fail safe real time clock in a programmable clock, particularly a thermostat incorporating a vacation mode of operation.